SlB-NuMBER AND OTHER FAMILY-SlZE EFFECTS*
The number of siblings in a family is likely to be important with respect to at least two elements of nurture: nutrition (16, 17) and preschool training. It may also be important in terms of the existence, or absence, of maternal deprivation, but this is not at all clear. With respect to all three of these effects there is some impressive case study evidence although we do not know how widespread these cases are from a statistical viewpoint. Other things being equal, we would normally expect that the number of siblings will determine the nutrition of children and hence the greater the sibling number, the greater the likelihood of malnutrition in low income families (16, pp. 142-143).
For example, J. A. Scott (18) studied a cross-section of children attending ordinary day schools in London. He collected data in 1959 on height and weight of pupils and then linked the results of their "eleven-plus" examinations (verbal reasoning test) with the data. Table 1 shows that as the number of children in the family increases, mean height, weight, and intelligence scores tend to decrease. (Two exceptions—intelligence of a two-child family in 1G and + height + weight of a four-child family in IB.) Conversely—". . . children who belong to small families tend to do better in intelligence tests than children from larger families, and that children from large families are not so tall (or so heavy) at any given age as those from small families."
In Table 2 Scott shows that as average intelligence increases so does the average height. Table 1 shows that intelligence is related to family size and that height is also related to family size. The question is raised as to which of these two variables (height or family size) has the greater influence on intelligence. Table 3 shows that intelligence increases with height and decreases with family size.
Scott therefore says the data suggest that the most intelligent child will be found in the small family.
Similarly, it has been shown that the greater the sibling number the less the effectiveness of informal preschool training on linguistic skills or I.Q. (17, p. 130).t
*Sec also J. D. Wray, "Population Pressure on Families: Family Size and Child Spacing," in this volume, especially the tables.
tlhere is a large literature on these matters which shows an inverse relationship between intelligence and aspects of family size such as sib number, etc. See (10, 11). Results are questioned by Blackburn (18). See the symposium edited by Scrimshaw and Gordon (19), with report of experiment by Harold Skeels, pp. 353-354. See also Patton and Gardner (20), with report by Thomas, Springfield, Illinois, 1963, in which six cases are cited in which extreme maternal deprivation has been associated with retardation of physical growth and delayed skeletal maturation. However, this last may be the consequence of the accompanying malnutrition rather than maternal deprivation as such.